vsh: Cmd aliases lookups should return results for the aliased command

vsh: Cmd aliases lookups should return results for the aliased command

Unfortunately, we have a number of aliases in virsh and even though
these are not visible any more, we have to support them. The problem is
that when trying to print help for the alias, we get SIGSEGV because
there isn't any @def structure anymore and we need to query the command
being aliased instead.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1538570

Signed-off-by: Erik Skultety <eskultet@redhat.com>